The proposed research focuses on a central problem of social cognition for all ages-how does the brain encode others' actions? This problem underlies typical as well as atypical (e.g., autism spectrum disorder; ASD) development of social learning and action perception. The answer is hypothesized to lie in the brain's neural mirroring properties found in the mirror neuron system and the EEG mu rhythm. The mu rhythm responds (mu rhythm desynchronization; MRD) both when executing a goal-directed action and when observing someone else perform the same action (Muthukumaraswamy & Johnson, 2004). This research focuses on how the infant brain encodes others' actions. Our primary goal is to characterize the development of MRD (during action observation/execution) between 1.5 and 6 postnatal months. Although there is a growing infant MRD literature, this research has focused on older infants and the few studies of MRD's mirroring properties include infants 9 months and older (Southgate et al., 2009), leaving significant gaps in the literature. Further, understanding of infant MRD development is limited because studies have used different ages, methodologies, and analyses (Cuevas et al., 2014; Marshall & Meltzoff, 2011, for review), which impede direct comparisons of findings across ages and across studies. Our secondary goal is to examine whether individual differences in infant MRD are related to other aspects of development, including motor, language, imitation, and social-emotional development. There is evidence that (a) motor experience is related to mu reactivity (van Elk et al., 2008); (b) language comprehension and speech perception are related to neural mirroring activity in adults (LeBel et al., 2009, for review); and (c) individual differences in MRD are associated with imitation skills for adults and children with ASD (Bernier et al., 2007, 2013). Further, social learning is theorize to serve both socio-emotional (e.g., cooperation, communication) and cognitive (e.g., acquisition of motor skills, causal information, and tool-use skills) functions (Marshall & Meltzoff, 2011) tha are essential for optimal behavioral and cognitive development. We will investigate (a) factors that affect or are related to MRD (facial vs. manual gestures; motor skills); (b) the potential use of MRD as an indicator of social information processing and/or language development; and (c) age-related changes (6-9 weeks; 6 months) in MRD. These data will be unique because they will be the first attempt to (a) examine MRD in 6-week-olds; (b) test human infant facial MRD; (c) analyze infant MRD development longitudinally; and (d) investigate both concurrent and predictive infant MRD associations (i.e., motor, language, social-emotional, imitation skills). Our findings will have implications for theories of cognitive and social development as well as optimal MRD methodology during infancy. These results will also provide clinical researchers with a typically developing reference, which could potentially serve as an early neurological indicator of social information processing deficits (e.g., ASD) and other communicative disorders that would be apparent before behavioral or language indicators.